In order to obtain a favourable combustion, including an efficient destruction of soot, neutralization of acid combustion residues and recovery of residual heat, the flame from an oil burner (53) is directed axially into a tube (52) of refractory material. At the exit from the tube there is a porous slab (54) of refractory material which, in use, is maintained in incandescent state, so soot will be consumed. The soot-free combustion gases are transferred into a column (60) in the upper portion of which a neutralizing agent containing fluid is sprayed through a nozzle (65). The fluid is collected in the lower portion (79) of the column for recirculation to the nozzle (65) by means of a pump (63). The neutralizing agent is added by means of a metering device (67), and a heat recovering coil (70) is provided in the fluid containing, lower portion (62) of the column.